1. Field of the Invention
The present invention relates to an improved apparatus for determining the charge of aqueous suspensions of finely divided, solid, charged particles.
2. General Background
Coagulation and flocculation are frequently employed to clarify aqueous solutions containing suspended particles. Once coagulated and flocculated, the suspended particles can be separated from their fluid medium by sedimentation, filtration, flotation, or centrifugation. Coagulation is promoted by the use of chemicals such as alum, ferric chloride or various polymeric materials such as water-soluble cationic and anionic organic polyelectrolytes. The polymeric materials are also used as flocculents. Flocculation is used for example, in a paper machine head box within a paper manufacturing process operation and in many other contexts.
U.S. Pat. Nos. 3,368,144 and 3,368,145-Gerdes describe a detector which develops a signal which can be used in the continuous regulaton of flocculation of aqueous suspensions of finely divided charged particles and for other uses, such as detecting electrical charges present in a stream having ions, charged molecules, or colloidal particles therein. In the Gerdes apparatus, a magnetically actuated mechanical switch is used to synchronously detect the alternating current signal developed between two electrodes as a piston moves a fluid sample containing charged particles past the electrodes in periodically alternating directions.
U.S. Pat. Nos. 3,369,984, 3,399,133 and 3,526,827 also describe prior art detectors.
U.S. Pat. No. 4,446,435-Canzoneri, describes an improvement on the streaming current detector of the Gerdes patent. As noted in column 6, line 45 et seq. of the Canzoneri patent, the mechanical switch used in the prior art to commutate the alternating current signal, was a common cause of failure. As shown in the Canzoneri patent, the mechanical switch was eliminated and replaced by an optical isolator. The optical isolator of Canzoneri consisted of a light emitting diode source and photodiode light detector. The light emitting diode source generated and directed a beam of light onto the reflective surface of a rotating disk and the photodiode detector sensed the presence of the reflected light. A non-reflective mark on the disk periodically interrupted the beam of reflected light from the disk which was sensed by the photodiode detector. The resultant photodetector output signal was synchronized with the stroke of the piston and was used to control the periodic sampling of the voltage from the electrodes.
Problems have been encountered in the use of reflective optical isolator devices of the type shown in Canzoneri. One problem is their inherent sensitivity to stray light and marginally reflective surfaces. These two characteristics can cause erroneous output signals resulting in incorrect signal detection.
Another problem encountered in prior art streaming current detectors is that the amplifier for the electrodes is sensitive to sample fluid parameters other than charge and does not accurately measure the charge condition of the fluid under all conditions. In addition, prior art streaming current detector amplifiers are sensitive to input cable capacitance making system calibration and repair difficult to accomplish.
It is an object of the present invention to provide an improved streaming current detector which obviates these and other problems.